On July 21, 1970, Chicago performed a live concert at Tanglewood, showcasing their evolving sound during a pivotal period in their career. By this time, they had released their second studio album, "Chicago II," earlier in 1970, which continued to build on their fusion of rock and jazz. "Chicago II" featured notable hits like "25 or 6 to 4," which became a defining track for the band. The live performance at Tanglewood came at an essential moment when Chicago was solidifying their identity in the music scene, transitioning from their debut album's success to greater experimentation with their sound. The concert highlights their energetic stage presence and instrumental prowess, showcasing their commitment to live performance, setting a standard for their subsequent shows. This era marked the beginning of Chicago's trajectory toward becoming one of the most successful rock bands of all time, blending intricate arrangements and brass instruments in a captivating way.